Fly Baby Rigging and Trim

December 2012

The terms "Rig" and "Trim" are different, but are really
related. Most folks refer to "Trim" as the feel of the
aircraft, but it's also used for the devices that may adjust
that feel. But really, it's the Rig of the aircraft that
affects how it flies and whether you need to adjust the trim.

Rudder Trim

Light airplanes in the Fly Baby's class don't usually have
cockpit-adjustable rudder trim. Many have exterior tabs,
like you can see in the photo below.

Some folks seem to think that having to add a rudder tab is a
negative reflection on their workmanship. "They guy didn't
build it straight, look, he had to add a fixed tab to the
rudder."

But the Fly Baby has a good excuse. Many planes build in a
slight offset to the vertical stablizer to compensate for engine
torque (oh, OK, "P-Factor"). But Pete wanted the Fly Baby
to be adaptable to a wide variety of engines, some of which turn
in the "wrong" direction. Having the vertical stab offset
in the wrong direction wouldn't do anyone any good, so Pete's
building instructions construct the stabilizer with no
offset. So, unless you install a jet, you WILL probably
have to install a fixed rudder tab.

It's pretty darn simple...just look at the
picture above. It's a piece of thin aluminum (~0.040"),
six or eight inches long and ~2 1/2" wide, screwed onto the
rudder's trailing-edge bow with a couple of #6 or #8
screws. This is my own airplane, and the size seems to
work just fine.

Which way to bend it? Remember, the trim tab acts as an
"anti-servo", so if you want to force the rudder to the left,
you have to deflect the tab to the right.

Which side? Well, the mighty Continental tries to force
the nose to the right, so the tab will need to be bent to the
left. If you install the tab on the right side of the
rudder, that gives it a bit of "left bend" right from the
start.

"But....but... Ron, yours is installed on the left!"

Well, yes. I have a good reason for that: Somebody
had already put holes on the left side of the rudder bow.
Rather than drill a couple of new holes and leave the old
unslightly ones in view, I installed the tab on the left side.

Plus... well my diagram on the right notwithstanding, the
trailing edge of the rudder and elevator on a Fly Baby is pretty
blunt. I'm thinking that the stagnation area behind the
tailing edge might make a tab installed on the "correct" side
less efficient.

It doesn't make a lot of difference. The tab is really pretty
small, and it's not going to make you snap-roll on takeoff if
you don't get the position, size, or bend right. Just
install it without bending it, then go fly. Get a feel for
whether you have to hold rudder at cruise. Then land, bend
the tab a bit, and fly again. Pretty simple.

Elevator Trim

It comes as a surprise to some folks that the Fly Baby does not have cockpit adjustable
trim. Folks get so used to the high control
pressures required for your typical Cessna or Piper, that the
whole concept of NOT having to adjust the trim with every
airspeed change is alien to them.

The fact is, the Fly Baby trim doesn't change much, in
flight. We've got only one seat (so the load doesn't
change much), the speed range is only 50 MPH or so, and the
elevator pressures are light enough so that out-of-trim
conditions aren't too noticable. We can get by with fixed tabs,
adjusted to a good compromise setting.

Install a fixed tab as shown for the rudder. I adjusted
mine so that the plane is slightly nose-heavy when the fuel tank
(which is slightly forward of the CG) is full. When the
tank gets towards empty, the plane becomes slightly tail-heavy.

I don't find the amount of pressure is a problem. However,
if someone builds their plane for long-distance cruising or is
more sensitive than I am, they can install a trim system if they
wish. Chuck Baynard decided he wanted cockpit-adjustable
trim on his Fly Baby, and installed one. He supplied a nice writeup on how he did
it. His is a spring-type system similar to a number of
production-type aircraft.

I suggest you build your plane without a trim system, and add
one like Chuck's later if you really feel the lack.

Aileron (Wing
Heaviness)

Do you have to hold the stick to one side or the other during
cruise flight? If so, you might need to perform a
gen-u-wine adjustment to the Fly Baby's rigging. I did,
several years back.

Wing heaviness can be caused by a variety of things. The
wing rigging could just be wrong, one wing may actually be
heavier than another, a yaw tendency is being translated into a
wing heaviness, or the two wings could even be built
different. I once saw a Mini-Mustang where the profiles of
the leading edges did not match...one was obviously sharper!
Probably made it stall strange, too.

You could
put a fixed tab on an aileron, but there's a lot cleaner, easier
way. The wing wires on a Fly Baby are used to set the
wash-in/wash-out for each wing, and you can easily fiddle with
the bracing to get the plane to fly the way you want it
to. You actually twist the wing to slightly increase or
slightly decrease its lift. This is the way all aircraft
had their trim adjusted, until cantilever wings started showing
up.

To make this adjustment, you'll need an awl or a stout nail, a
spool of safety wire, and a pair of side cutters.

There are two ways to proceed: You can reduce the angle of
incidence of the "Light" wing, or increase the angle of the
"Heavy" wing. Let's assume you're going to adjust the
heavy wing to produce more lift.

Go to the Heavy wing with your tools and crawl underneath the
trailing edge of the wing near the inboard end of the
aileron. You'll be looking up at the pair of turnbuckles
that attach to the BACK flying wire point.

Pretty easy, here: Remove the safety wire from both
turnbuckles. Then, stick your awl or nail into the center
hole of the turnbuckle, and tighten it one or two turns.
It's sometimes hard to tell which way to turn it. After it
starts turning, though, if it's getting tighter (the right way),
it get stiffer to turn. Also the wire attached to the
parallel flying wire will seem to get a bit slack.

After you've tightened one turnbuckle one or two turns,
safety-wire it, and then tighten the OTHER turnbuckle the same
amount. Safety-wire it, and you're ready for a test
flight!

If it gets so that the turnbuckle won't tighten, you need to
slack off a bit on the corresponding landing wire on top of the
wing. Those turnbuckles are right outside the
cockpit. Remove the safety wire, slack them off a turn or
two, then re-safety them.

Remember to use fresh safety wire each time (e.g., don't reuse
it).

IIRC, I did ~2 turns on my first shot, and it helped my slight
wing-heaviness but didn't fully take it out. Took another
one or two turns. One or two turns is pretty gradual, you
shouldn't wildly affect the handling.

If your aft flying wire turnbuckles are already tightened all
the way...well, then, adjust the OTHER wing the opposite
way: Tighten the leading flying wires to reduce its lift
slightly.

This is really a pretty subtle set of changes. Other than
the wing-heaviness disappearing, you probably won't notice any
difference to the aircraft.